There is known a tandem type image forming apparatus which comprises a plurality of image forming sections for forming a color image.
In this type of image forming apparatus, it is important to improve the image quality to reduce the deviation of the images (hereinafter referred to as element image) formed by each of the plurality of image forming sections as much as possible. Thus, the parameters relating to the operations of each of the plurality of image forming sections are properly adjusted.
One of the parameters mentioned above is horizontal scanning magnification. The change of the horizontal scanning magnification has an impact as the change in the size of the element image in the horizontal scanning direction.
Thus, a test image having a given pattern nearby two ends of an image forming range in the horizontal scanning direction is formed by each of a plurality of image forming sections, and the deviation of the horizontal scanning magnification is measured according to the deviation of the position relation between two patterns in the test images. In order to detect the deviation of the position relation between two patterns in the test images, it is necessary to arrange two sensors nearby each of two ends of the image forming range in the horizontal scanning direction.
Thus, it is preferred to reduce the number of the sensors for adjusting the horizontal scanning direction magnification to one.